The effects of current methods of bone marrow transplantation (BMT) on pulmonary function in children have not been extensively studied. We reviewed serial pulmonary function tests (PFTs) in 25 children (median age, 9 yr; range, 4-15) who received allogeneic (n = 14) or autologous (n = 11) BMT for neoplastic diseases at The Johns Hopkins Hospital. The PFTs were obtained before BMT and at 6 months (early) and 15 months (late) after transplant. In all but 6 patients, PFTs were normal before BMT. A mild transient decline in carbon monoxide diffusing capacity (DLCO) was observed early after BMT but returned to baseline levels in the late post-BMT period. A trend towards worsening of PFTs with increasing age of patients was observed. The presence of graft-versus-host disease (GVHD) and pretransplant seropositivity for cytomegalovirus (CMV) were associated with significant decrements in several measurements of pulmonary function in the early post-BMT period. Patients given bulsulfan-containing preparative regimens tended to have less impairment of PFTs than those given other regimens using other combination high-dose chemotherapeutic agents or total body irradiation. These findings suggest that abnormalities in PFTs are common in the first months after BMT in pediatric patients but are not consistently associated with impairment of lung function when studied 15 months post-transplant.